TT #169 · Parshas Tazria · Aliyah 4 · Passuk 24
English: If the flesh has a burn from fire in its skin, and the raw area of the burn becomes a bright reddish-white spot or white,
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:24
Torah Temimah cites Sifra (Toras Kohanim) for the rule that a boil and a burn do not combine for the qualifying measure.
Sifra — Vayikra 13, Midrash Sefer
English: A boil and a burn do not combine with one another.
Textual derivation
A boil and a burn are separate categories; their areas cannot be added together to meet the required measure.